agreement: don't attempt to make proposal if no particating accounts

[cce]

Summary

As described in #590 if you don't have any participation keys, you don't need to make a proposal. This adds a check to skip calling AssembleBlock() if there are no participating accounts.

Test Plan

Existing tests should pass, also need to write new test asserting AssembleBlock is not called.

[codecov]

Codecov Report

Attention: Patch coverage is 47.05882% with 9 lines in your changes missing coverage. Please review.

Project coverage is 53.54%. Comparing base (a6eece5) to head (1902e59).
Report is 768 commits behind head on master.

Files with missing lines	Patch %	Lines
data/pools/transactionPool.go	20.00%	3 Missing and 1 partial ⚠️
agreement/pseudonode.go	0.00%	1 Missing and 1 partial ⚠️
node/node.go	0.00%	2 Missing ⚠️
data/account/participationRegistry.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4976      +/-   ##
==========================================
- Coverage   53.56%   53.54%   -0.02%     
==========================================
  Files         430      430              
  Lines       54095    54107      +12     
==========================================
- Hits        28978    28974       -4     
- Misses      22869    22884      +15     
- Partials     2248     2249       +1     

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